Urgent help-- Requester Change request failing

suddenly we are having issue in our application where requester change process is failing with inactive buyer however buyer is active and no issue with his/her account. 

After the internal approval of requester initiate change, system sends notification to buyer to accept or reject.  Buyer accepts notification however system says the change has been rejected with below exception.  Its not reproducible and not happening for all buyers although for same buyer its not happening for all documents.

Exception when processing the change request : Buyer is inactive. Please update the document with a valid buyer.

The issue is growing but not getting some good direction from Oracle SR.  If you have any idea or clue which can help to find the cause.